The present invention relates to a numerical control method. More particularly, it relates to a numerical control method for a numerical control system having a mirror image function.
A numerical control (NC) system has a mirror image function wherein the moving direction of a movable member, such as a tool or a table, is made the reverse of a commanded direction entered from an NC tape or the like. In general, the NC system has a memory for storing the coordinate values of a commanded position and the coordinate values of a current machine position. Irrespective of whether the mirror image function is in effect or not, the commanded position coordinate values stored in the memory are updated in the direction of an input command each time distributed pulses are generated. On the other hand, the current machine position coordinate values stored in the memory do depend upon whether or not the mirror image function is in effect. They are updated in the direction opposite to that of the input command for the effective mirror image function and in the same direction as that of the input command for the ineffective mirror image function each time the distributed pulses are generated. Thus, they indicate the current machine position at all times.
In a prior-art NC system, both the commanded position coordinate values and the current machine position coordinate values are updated each time a numerical command is given on the basis of which pulse distribution calculations are executed to generate distributed pulses. Simultaneously updating the two series of coordinate values during the pulse distribution in this manner leads to an increase in processing load and thus poses a major problem especially for an NC system which relies on a built-in microcomputer having low operating speed. It is sometimes impossible to update the coordinate values in the interval from one step of generating distributed pulses to the next step of generating distributed pulses. Particularly when the commanded speed is high, the time interval between the pulse generating steps is short. Moreover, processing steps other than those for coordinate value renewal must be performed in at the same time. Therefore, a situation arises wherein the coordinate value update does not take place in time and this is a significant source of trouble.